Dual dielectric structure for suppressing lateral leakage current in high fill factor arrays

ABSTRACT

A structure and method for suppressing lateral leakage current in full fill factor image arrays includes dual dielectric passivation layer. A first passivation layer includes a material that is an insulator, has a low dielectric constant to minimize capacitive coupling between the contacts, and is low stress to prevent cracking. A second passivation layer includes a thin oxide or nitride layer over the first passivation layer.

This is a division of application Ser. No. 09/419,293, filed Oct. 15,1999, now U.S. Pat. No. 6,384,461 and which is incorporated herein byreference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

This invention was made with United States Government support underAgreement No. 70NANB7H3007 awarded by NIST. The United States Governmenthas certain rights in this invention.

FIELD OF THE INVENTION

The present invention relates generally to the structure of a full fillfactor image array and its method of manufacture, and more particularlyto the structure of a full fill factor image array that reduces lateralleakage current and its method of manufacture.

BACKGROUND OF THE INVENTION

A conventional image array is typically formed of a plurality ofphotosensitive elements or pixels arranged in rows and columns. Aconventional photosensitive element is shown in FIG. 1. Eachphotosensitive element (10) comprises photosensitive island (12) ofamorphous silicon (a-Si) over a bottom contact pad (14) over a substrate(11). A transparent upper conductive layer (16) of indium tin oxide(ITO) resides over the assembly. A passivation layer (18) is disposedunder the ITO (16) except where the ITO is in electrical contact withthe upper surface of the of the photosensor island (12). Thephotosensitive element further includes a N+ doped region (22) and a P+doped region (24). A conventional passivation layer (18) comprises anoxyinitride layer (26) and a polyamide layer (28).

In contrast to the conventional image arrays in which each pixel isdefined by a stand alone PIN photosensitive element, the full fillfactor image array improves the pixel fill factor by using continuouslayers of amorphous silicon (a-Si) and P+ doped a-Si. The pixels in ahigh fill factor array are defined only by a mushroom shaped collectionelectrode.

A conventional full fill factor image array is shown in FIG. 2. The fullfill factor image array (40) includes a plurality of source-drain metalcontacts (44) on a substrate (42). For ease of description, otherelements that may be formed on the substrate, such as gate lines, datalines, and thin film transistors (TFTs) are not shown. The source-drainmetal contacts (44) are formed of an electrically conductive materialsuch as a molybdenum and chromium alloy. They typically comprise one ormore layers of metal. The source-drain metal contacts (44) areelectrically connected to the switching and processing circuits (notshown).

To increase the area of carrier collection, a patterned mushroom-shapedcollection electrode (46) is provided over the source-drain metal (44).Disposed on top of the mushroom shaped collection electrode is an N+doped a-Si layer (48).

In contrast to a conventional image array using stand alone PINphotosensitive elements, the full fill factor image array uses acontinuous intrinsic amorphous silicon (i a-Si) layer (50) and acontinuous P+ doped a-Si layer (52). An upper electrode (54) of ITOresides on top of the P+ doped a-Si layer.

A conventional passivation layer comprises an oxynitride layer (56)deposited to thickness of about 1 micron by plasma enhanced chemicalvapor deposition (PECVD). The passivation layer serves as the insulationbetween the source-drain metal (44) and photosensor (46, 48, and 50).

Like conventional photosensitive elements, the sensor structure ofconventional full fill factor image arrays suffers from intrinsicleakage. Intrinsic leakage (58), represented in FIG. 2, arises due tomaterial defects rather than the structure of the photosensitiveelement. The intrinsic leakage current of a 60×60 μm² stand alone PINsensor, for example, is about 2 femto-amps (fA). In addition tointrinsic leakage, however, the conventional full fill factor imagearray structure suffers lateral leakage between pixels. This lateralleakage current causes image blurring effects and severely reduces theimager performance. The lateral leakage current for a 60×60 μm² fullfill factor array may be as high as 0.3 pico-amps (pA) at 5 V.

The bulk conductivity of high quality intrinsic a-Si is less than 10⁻¹¹(Ω·cm)⁻¹ which is about 2×10¹⁴ Ω between a 60×60 μm² pixel and itsneighbor. Therefore, the intrinsic bulk conductivity of a-Si cannot beresponsible for the high lateral leakage current. The most likelyconducting mechanism for this lateral leakage current is conductionthrough the accumulated charge in the a-Si and oxynitride interface.Both the trapped positive ion in the oxynitride and the interface statesin the a-Si/oxynitride interface can cause electron accumulation in thea-Si/oxynitride interface, creating a conducting channel between pixels.

One solution is to replace the passivation layer with another dielectricmaterial such as silicon oxide or silicon nitride. Deposition rates forsilicon oxide or silicon nitride, however, are usually much lower thanfor oxynitride. Therefore, deposition of a silicon nitride or siliconoxide passivation layer of about 1 micron would be impractical. Inaddition, other problems such as stress build-up may degrade the sensorstructure.

In light of the foregoing, there is a need for a method and structure toreduce the lateral leakage current in full fill factor sensor arrays.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to a high fill factorimage array including a plurality of source-drain metal contactsdisposed in an image array pattern, a dual dielectric passivation layerthat suppresses lateral leakage current comprising a first passivationlayer and a second passivation layer deposited over the firstpassivation layer, wherein the thickness of the second passivation layeris less than the thickness of the first passivation layer, a continuouslayer of a-Si, a plurality of a patterned collection electrodes disposedon top of the source-drain metal contacts, a first doped silicon layerdisposed over the collection electrodes a continuous second dopedsilicon layer, and an upper electrode.

In another aspect, the invention is directed to a method for making ahigh fill factor image array including the steps of providing aplurality of source-drain metal contacts, depositing a first passivationlayer, depositing a second passivation layer that suppresses lateralleakage current, opening a plurality of via holes through the first andsecond passivation layers, depositing a layer of conductive material,depositing a first doped a-Si layer, patterning to form the collectionelectrodes, depositing a continuous layer of i a-Si, depositing acontinuous second layer of doped a-Si, depositing and patterning anupper conductive layer.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and areintended to provide further explanation of the invention as claimed.

The accompanying drawings are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate several embodiments of theinvention and together with the description serve to explain theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate an embodiment of the inventionand, together with the description, serve to explain the objects,advantages, and principles of the invention.

FIG. 1 is schematic cross sectional view of a prior art PINphotosensitive element.

FIG. 2 is schematic cross sectional view of a prior art full fill factorsensor array.

FIG. 3 is schematic cross sectional view of a dual dielectricpassivation structure in a full fill factor sensor array according toone embodiment of the present invention.

FIG. 4 is a plot of current versus voltage that shows the measuredleakage current for a conventional sensor array and a sensor array witha dual dielectric passivation layer.

FIGS. 5 a-d are cross-sectional views that schematically illustrates thesteps in making the image array with the dual dielectric passivationlayer according to one embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Reference will now be made in detail to the preferred embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings.

FIG. 3 illustrates the components of a full fill factor sensor arrayformed in accordance with the principles of the present invention.According to one aspect of the invention, the fundamental components ofan a full fill factor sensor array (140) include a plurality ofsource-drain metal contacts (144) arranged in rows and columns, aplurality of patterned mushroom shaped metal contacts (146) over thesource-drain metal contacts, N+ doped a-Si regions (148) over themushroom shaped collection electrodes (146), and a dual passivationlayer (156, 157). The full fill factor sensor array further includescontinuous layers of i a-Si (150) and P+ doped a-Si (152). An upperelectrode layer (154) of, for example, ITO is disposed on top of the P+a-Si layer.

The dual passivation layer (156, 157) includes a first dielectric layer(156) of a material that is an insulator, has a low dielectric constantto minimize capacitive coupling between the contacts, and is low stressto prevent cracking. Examples of passivation materials include siliconoxynitride, benzo-cyclo-butene (BCB), or polyimides. The thickness ofthe first passivation layer (156) depends on the material used and canbe, for example, 1 to 5 μm. A first passivation layer (156) ispreferably oxynitride with a thickness of about 1 micron.

A second dielectric layer (157) of, for example, an oxide or nitride isthen deposited over the first dielectric layer. The thickness of thesecond dielectric layer (157) is not critical, but should be thinnerthan the first passivation layer. Preferably, the second passivationlayer (157) should be a oxide with a thickness of about 1000 Å.

The dual dielectric passivation layers' (156, 157) effect on the lateralleakage current is demonstrated in FIG. 4. Lateral leakage current wasdirectly measured on test arrays comprising 60 μm square pixels, 20 μmapart. In one test array a 1 micron thick oxynitride passivation layerwas deposited. In a second test array, a dual dielectric passivationstructure comprising a 1 micron thick oxynitride layer and a highquality oxide layer with thickness of about 1000 Å was deposited. Inorder to isolate the lateral leakage current out of the total leakagecurrent which includes intrinsic leakage current and lateral leakagecurrent, the P+ a-Si and ITO layers were not deposited on either testarray. As shown in FIG. 4, the test array with the conventionaloxynitride passivation layer showed a lateral leakage current as high as0.3 pA at 5 volts. The measured lateral leakage current for the dualdielectric passivation structure according to one embodiment of thepresent invention is about 4 fA at 5V. Thus, the dual passivation layerstructure reduces lateral leakage current by about two orders ofmagnitude over the conventional structure.

The method of making the image array with the dual dielectricpassivation layer according to one embodiment of the present inventionis shown in FIGS. 5 a-e. As shown in FIG. 5 a, the first passivationlayer (156) is deposited over the source drain metal contact (144) and asubstrate (not shown) that includes gate lines (not shown), data lines(not shown), and switching circuits such as, for example, thin filmtransistors (TFT). The thickness of the first passivation layer (156)depends on the material used and can be deposited, for example, to athickness of about 1 μm. An oxynitride layer can be deposited by, forexample, plasma enhanced chemical vapor deposition (PECVD). A BCB layercan be deposited by, for example, spin coating or spray coating.

A second passivation layer (157) is then deposited over the firstpassivation layer. The second passivation layer (157) reduces thelateral leakage current and is, for example, a high quality oxide ornitride. The thickness of the second passivation layer (157) is notcritical, but should be less than the first passivation layer. Thesecond passivation layer is preferably an oxide with a thickness ofabout 1000 Å. It is preferably deposited by PECVD.

As shown in FIG. 5 b, vias (160) are then opened to expose thesource-drain metal (144). Via holes can be opened, for example, byetching through the first and second passivation layers. A metal layeris then deposited so that it contacts the source-drain metal contact.This is followed by deposition of a first doped a-Si layer (148). Thefirst doped a-Si layer (148) can be, for example, N+ doped a-Si. Themetal layer and the first doped a-Si layer (148) are then patterned, asshown in FIG. 5 c, such that the patterning of the metal layer forms themushroom shaped collection electrode (146) that defines the pixels.

Then, as shown in FIG. 5 d, a continuous layer of i a-Si (150) isdeposited. A second doped a-Si layer (152) is deposited over the a-Silayer. The second doped a-Si layer (12) can be, for example, P+ dopeda-Si. An upper electrode (154) of, for example, ITO is then depositedover the second doped a-Si layer. The ITO, a-Si, and P+ a-Si are thenpatterned, preferably in one lithography step.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the dual dielectricstructure and the method for suppressing lateral leakage current of thepresent invention without departing from the spirit or scope of theinvention. Thus, it is intended that the present invention cover themodifications and variations of this invention provided they come withinthe scope of the appended claims and their equivalents.

1. A method for making a high fill factor image array comprising thesteps: providing a plurality of source-drain metal contacts on asubstrate; depositing a first passivation layer over the plurality ofsource-drain metal contacts and the substrate; reducing the lateralleakage current between the plurality of source-drain metal contacts inthe high fill factor image array by depositing a second passivationlayer over the first passivation layer, the second passivation layerbeing thinner than the first passivation layer; opening a plurality ofvia holes through the first and second passivation layers to theplurality of source-drain metal contacts; depositing a layer ofconductive material over the plurality of source-drain metal contactsand the second passivation layer; depositing a first doped a-Si layer asan optically active layer over the layer of conductive material;patterning the first doped a-Si layer and the layer of conductivematerial to form collection electrodes; depositing a continuous layer ofi a-Si disposed on the second passivation layer and the first doped a-Silayer; depositing a continuous second layer of doped a-Si over thecontinuous layer of i a-Si; depositing an upper conductive layer overthe second layer of doped a-Si; and patterning to form the image array.2. The method for making a high fill factor image array according toclaim 1, wherein the first passivation layer comprises siliconoxynitride, BCB, or a polyamide.
 3. The method for making a high fillfactor image array according to claim 1, wherein the second passivationlayer is an oxide.
 4. The method for making a high fill factor imagearray according to claim 1, wherein the second passivation layer has athickness of about 1000 Å.
 5. A high fill factor image array formed by:providing a plurality of source-drain metal contacts on a substrate;depositing a first passivation layer over the plurality of source-drainmetal contacts and the substrate; reducing the lateral leakage currentbetween the plurality of source-drain metal contacts in the high fillfactor image array by depositing a second passivation layer over thefirst passivation layer, the second passivation layer being thinner thanthe first passivation layer; opening a plurality of via holes throughthe first and second passivation layers over the plurality ofsource-drain metal contacts; depositing a layer of conductive materialon the plurality of source-drain metal contacts and over the secondpassivation layer; depositing a first doped a-Si layer as an opticallyactive layer over the layer of conductive material; patterning the firstdoped a-Si layer and the layer of conductive material to form collectionelectrodes; depositing a continuous layer of i a-Si disposed on thesecond passivation layer and over the first doped a-Si layer; depositinga continuous second layer of doped a-Si over the continuous layer of ia-Si; depositing an upper conductive layer over the continuous secondlayer of doped a-Si; and patterning to form the image array.
 6. The highfill factor image array of claim 5, wherein the first passivation layercomprises at least one of silicon oxynitride, BCB, or a polyimide. 7.The high fill factor image array of claim 5, wherein the secondpassivation layer is an oxide.
 8. The high fill factor image array ofclaim 5, wherein the second passivation layer has a thickness of about1000 Å.
 9. A method for making a high fill factor image arraycomprising: providing a plurality of source-drain metal contacts;depositing a first passivation layer over the plurality of source-drainmetal contacts; reducing the lateral leakage current between theplurality of source metal contacts in the high fill factor image arrayby depositing a second passivation layer over the first passivationlayer being thinner than the first passivation layer; opening aplurality of via holes through the first and second passivation layersto expose the plurality of source-drain metal contacts; depositing alayer of conductive material on the plurality of source-drain metalcontacts, such that the layer of conductive material makes electricalcontact with the plurality of source-drain metal contacts; depositing afirst doped a-Si layer as an optically active layer of conductivematerial; patterning the a-Si layer and the layer of conductive materialto form collection electrodes; depositing sensor material comprising acontinuous layer of i a-Si over the collection electrodes and at least aportion of the second passivation layer; depositing a continuous layerof doped a-Si over the continuous layer of i a-Si; depositing aconductive layer over the continuous layer of doped a-Si; and patterningconductive layer to form an upper electrode.
 10. The method for making ahigh fill factor image array according to claim 9, wherein the firstpassivation layer comprises silicon oxynitride, BCB, or a polyamide. 11.The method for making a high fill factor image array according to claim9, wherein the second passivation layer is an oxide.
 12. The method formaking a high fill factor image array according to claim 9, wherein thesecond passivation layer has a thickness of about 1000 Å.